1. use a manifest constant wxNO_LEN instead of -1 for lengths everywhere

1. use a manifest constant wxNO_LEN instead of -1 for lengths everywhere
2. reimplemented UTF-16/32 conversions using To/FromWChar() API instead
   of MB2WC/WC2MB for Windows (or rather SIZEOF_WCHAR_T == 2 platforms),
   the first tangible result is that reading UTF-32BE text streams now
   works too
3. more off by 1 fixes

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38585 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775